It is intended to provide a novel compound having high antitumor activity and low toxicity to normal cells. The present invention provides a pyridone carboxylic acid derivative represented by the following formula (1) or a salt thereof wherein R1 represents a hydrogen atom, a halogen atom or the like； R2 represents a hydrogen atom, a halogen atom or the like； R3 to R6 each represent a hydrogen atom or the like； R7 represents a hydrogen atom or the like； R8 represents a hydrogen atom, a halogen atom, the following formula (a) (wherein Ra1 and Ra2 each represent a hydrogen atom, a hydroxy group, an optionally substituted lower alkyl group or the like) or the like, or R7 and R8 together represent =N-OR10 (wherein R10 represents a hydrogen atom, an optionally substituted lower alkyl group, or an aralkyl group), or R7 and R8 form an optionally substituted 4- to 6-membered saturated hetero ring together with the adjacent carbon atom, or the like； R9 represents a hydrogen atom or the like； X represents a nitrogen atom or the like； and Y represents a nitrogen atom or the like.
